Numerous types of trays made from paperboard have been employed in the past for packaging food products. Some trays are formed from a unitary, paperboard blank using a drawing process. The tray may include an outwardly extending flange around the entire upper edge which provides the dual function of rigidifying the tray and providing a flat surface to which a plastic cover sheet may be bonded in order to enclose the top of the tray after the tray has been filled with food.
More recently, the depth of trays of the type described above has been increased in order to accommodate various types of food and increase the capacity thereof. Because these deeper trays are drawn from a single sheet of paperstock, it became necessary to pre-score the blank at the corners thereof in order to produce evenly distributed folds in the paperboard at the corners of the tray during the drawing process. These folds reduce stress concentrations in the corners while the paperboard is being deformed during the drawing process and thereby eliminate potential tearing of the paperboard stock. The folds in the paperboard created by pre-scoring thereof are present in the side walls of the tray and extend into the flange. The folds created in the flange result in alternating ridges and depressions forming a ribbed effect in the flange at the corners of the tray. The ribbed surface of the flange at the corners of the tray creates difficulty in bonding and completely sealing a cover sheet around the entire perimeter of the flange. Incomplete sealing of the cover sheet to the flange may result in some loss of the product from the tray during shipping, handling and display and may cause degradation of the food product due to exposure thereof to the surrounding environment.
Another problem related to sealing of a cover sheet to prior art type trays involves the fact that during filling of the tray with food product, portions of the food product, whether liquid or solid, may splash onto the flange. Food product remaining on the flange contributes to incomplete or weak sealing of the cover sheet to the flange.
Accordingly, it is an object of the present invention to provide a deep-drawn paperboard tray having prescored corners forming ridges in the flange thereof which includes troughs debossed into the irregular flange areas at the corners of the tray to promote an improved bond through the use of ultrasonically vibrating tooling designed to match the shaped of the troughs, when sealing a cover sheet to the flange of the tray.
Still another object of the present invention is to provide a novel method of sealing the cover sheet to the tray.
These and further objects of the invention will become clear or made apparent during the course of the following description. In accordance with the present invention, an improved tray particularly suited for packing food is deep drawn from a unitary, pre-scored blank of paperboard and includes an outwardly extending flange along the upper edge of the tray to provide a face to which a plastic cover sheet is bonded. After the tray is formed, arcuately shaped troughs are formed in the face of the flange at each corner of the tray by die pressing thereof. Simultaneous with the forming of the troughs, ridges existing in the flange created by folds in the paperboard at the scorelines therein are flattened by die members to provide a smooth bonding surface on the flange. A plastic cover sheet for enclosing the top of the tray is ultrasonically bonded to the trough areas of the flange after the tray is filled with food. The ultrasonically vibrative tooling clears the trough areas of extraneous food and the troughs can divert some of those foods spilled in those areas during the filling process back into the interior of the tray. Heat is subsequently applied to the total flange after ultrasonic bonding thereof to assure complete sealing of the cover sheet to the flange.